Microwave-Assisted Preparation and Hydrazine Decomposition

The WCx/CNTs were typically synthesized from CNTs (0.1 g) and tungsten ... So the tungsten loading (x%) can be calculated by the formula. where m0 and...
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Ind. Eng. Chem. Res. 2009, 48, 3244–3248

Microwave-Assisted Preparation and Hydrazine Decomposition Properties of Nanostructured Tungsten Carbides on Carbon Nanotubes Changhai Liang,*,† Ling Ding,† Aiqin Wang,‡ Zhiqiang Ma,† Jieshan Qiu,† and Tao Zhang‡ State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian UniVersity of Technology, 158 Zhongshan Road, P.O. Box 49, Dalian 116012, China, and State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, PO Box 110, Dalian 116023, China

Nearly monodispersed nanostructured tungsten carbide particles on carbon nanotubes (CNTs) have successfully been synthesized by microwave-assisted metal-organic chemical vapor deposition (MOCVD) at atmospheric pressure in a fluidized bed reactor. The results show that the tungsten carbide particles with 2-5 nm on CNTs can be formed several minutes and the particle sizes increase with the increase of microwave irradiation time. The preoxidation of CNTs is not necessary in the microwave-assisted MOCVD. The resulting materials are active catalysts for hydrazine decomposition and exhibit high selectivity to hydrogen, indicating that nanostructured tungsten carbides on CNTs is an inexpensive and promising alternative to the noble-metal catalysts for hydrazine decomposition. The microwave-assisted MOCVD is of great potential in the controlled synthesis of supported catalytic materials. 1. Introduction Catalytic hydrazine decomposition over noble metal iridium catalysts has been successfully used in altitude and orbit control of spacecraft.1-3 However, the traditional noble metal iridium with high loading (about 20-40 wt %) is very expensive and of limited availability. Therefore, it is highly desirable to develop inexpensive, active, stable, and readily available non-noble metal catalysts for the reaction. Transition metal carbides as catalytic materials have received considerable attention for their exceptionally high activities,4,5 which are similar to those of noble metal catalysts in hydrogeninvolving reactions.6-20 Tungsten and molybdenum carbides are being considered for use as inexpensive and promising alternatives to the traditional noble metal iridium with possibly even superior properties due to their ability to withstand high temperature and resist poisoning.6,7 As catalytic materials, nanostructured particles uniformly dispersed on supports and/ or high surface area are generally required in order to get high catalytic activity. The preparation of interstitial carbides with nanostructured particles and/or high surface area, however, is very difficult by the conventional methods which have been inherited from the metallurgical industry at high temperature, are energy-intensive, and result in large grains of low surface area. Although a variety of chemical and physical preparative methods have been developed, including pyrolysis of metal complexes,21-23 alkaline reduction in solution,24,25 temperatureprogrammed reduction,26,27 carbothermal hydrogen reduction,28-30 and sonochemical synthesis,31-33 the controlled synthesis of nanostructured carbides below a particle size of 5 nm still remains an important challenge for catalytic applications. Microwave techniques have been successfully used in organic synthesis and have been proven to be more environmental friendly which requires less energy than conventional processes. Recently, tungsten and molybdenum carbides from carbon and metal oxides or metals have been synthesized by microwave processing in tens of seconds.34-36 However, the carbides exist * To whom correspondence should be addressed. Fax: 86-41139893991. E-mail: [email protected]. † Dalian University of Technology. ‡ Dalian Institute of Chemical Physics.

as blocklike irregular crystallites with grain sizes typically ranging from 5 to 10 µm, and their morphology and grain sizes cannot be controlled. Compared with microwave processing, metal-organic chemical vapor deposition (MOCVD) has been shown to be a powerful method for generating highly dispersed and uniformly supported catalytic materials in a controlled and reproducible manner.37,38 Here, we have combined the microwave processing and MOCVD method to prepare evenly distributed tungsten carbide nanoparticles supported on carbon nanotubes (CNTs) in a fluidized bed reactor, which is an inexpensive and promising alternative to Ir catalysts for hydrazine decomposition. 2. Experimental Section The CNTs (Shenzhen Nanotech Port Co. Ltd.) were synthesized by CH4 decomposition over a nickel-based catalyst, and then, the catalyst was removed by dissolution typically in aqueous solution of HNO3. However, the CNTs are not welldefined multiwalled CNTs. The WCx/CNTs were typically synthesized from CNTs (0.1 g) and tungsten hexacarbonyl (0.383 g, Strem) by mixing in an agate mortar for 0.5 h. The CNTs were not preoxidised before use. The completely homogeneous precursor mixture was put in a quartz-tube reactor (inner diameter of about 10 mm) and fluidized with a flow rate of 30 mL/min argon for 2 h at room temperature. Then, the reactor was placed in a microwave oven operating at 2.45 GHz with a power of 800 W. The duration of microwave exposure was ranged from 3 to 30 min under argon. For comparison, WCx/ CNTs were also prepared by using the CNTs without pretreatment as support in a tubular furnace with thermal MOCVD under the inert atmosphere. The preoxidised CNTs were also used as the support in the microwave-assisted MOCVD. X-ray diffraction (XRD) analysis of the samples was carried out using a Rigaku D/Max-RB diffractometer with Cu KR monochromatized radiation source (λ ) 1.54178 Å), operated at 40 KV and 100 mA. The morphology and the particle size and distribution of the samples were studied by transmission electron microscopy (TEM, JEOL 2000, operated at 100 kV). Nitrogen adsorption and desorption isotherms at 77 K were measured using Micromertics 2010. Surface areas were calcu-

10.1021/ie801591x CCC: $40.75  2009 American Chemical Society Published on Web 02/17/2009

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Figure 1. XRD patterns of the samples prepared from microwave-assisted MOCVD.

Figure 2. XRD patterns of the samples prepared from thermal MOCVD.

lated from the linear part of the BET plot. The thermogravimetric/differential thermogravimetric (TG/DTG) experiments were also performed in Mettler Toledo TGA/SDTA851e thermogravimetry to obtain the tungsten loading. The samples were placed in the atmosphere of O2 and heated at 10 °C/min to the final temperature of 800 °C. Tungsten trioxide was the final product. So the tungsten loading (x%) can be calculated by the formula. x% )

m1 MW × 100% m0 MWO3

where m0 and m1 is the mass of the WCx/CNTs sample and final product, repectively; MW and MWO3 is the molecular mass of tungsten and tungsten trioxide, respectively. The catalytic activity and selectivity of the microwave-assisted MOCVD-produced WCx/CNTs samples were examined by hydrazine decomposition at atmospheric pressure in a fixedbed reactor. Catalytic hydrazine decomposition over the WCx/ CNTs was performed in a U-shaped quartz microreactor with 20 mg catalyst at the temperature ranging from room temperature to 800 °C. 3. Results and Discussion The typical XRD patterns of the WCx/CNTs and CNTs which were prepared by microwave-assisted MOCVD method from 15 s to 30 min are shown in Figure 1. The strong diffraction peaks at 26.5° observed in the diffraction of the WCx/CNTs can be attributed to the hexagonal graphite structures. The XRD pattern of the sample prepared with microwave irradiation for 15 s shows diffraction peaks of tungsten oxide at 25.74 and 37.04°, indicating that tungsten hexacarbonyl is first decomposed into tungsten oxide. When the irradiation time increases up to 3 min, two strong peaks were detected due to tungsten oxycarbide (face-centered cubic) at 37.1 and 42.8°, and a weak peak at 39.4°, which can be assigned to W2C with hexagonalclose packed (lattice parameters a ) 0.300 nm and c ) 0.473 nm), were also observed. No peak due to metallic tungsten or oxides was observed. With the increase of microwave irradiation, the reflection peaks due to tungsten oxycarbide become weaker and those corresponding to W2C become stronger. When the irradiation time further increases up to 20 or 30 min, three new diffraction peaks at 31.48, 35.78, and 48.38° were observed and attributed to WC with hexagonal-close packed structure

Figure 3. TG curves of the WCx/CNTs prepared from microwave-assisted MOCVD with the theoretic loading of 5, 10, 15, and 20 wt %.

(lattice parameters a ) 0.291 nm and c ) 0.284 nm). The average particle size of tungsten carbides is about 3.5 nm according to the Scherrer formula, which is much smaller than that prepared by the reaction between carbon and metals oxides or metals. When the thermal MOCVD route was employed, only WO2 was formed at 600 and 700 °C, as shown in Figure 2. By further increasing the temperature to 900 °C, a mixture phase of WO2 and W2C (hexagonal-close packed) was formed. The above results also indicate that formation of tungsten carbides under the microwave irradiation involves following phase transitions, i.e. tungsten hexacarbonyl f tungsten oxide f tungsten oxycarbide f W2C f WC. The transition from tungsten hexacarbonyl to tungsten oxycarbide was finished in tens of seconds. However, the direct thermal decomposition of tungsten hexacarbonyl leads to formation of WO2, W2C, and C even at 900 °C in the tubular resistance furnace. This confirms that the microwave-assisted MOCVD is efficient way for tungsten carbides. Under microwave irradiation, WO2 and C maybe react rapidly into tungsten oxycarbide. The formation of W2C was attributed to the reaction between tungsten oxycarbide and CO from decomposition of tungsten hexacarbonyl (above 170 °C). The transition from W2C to WC has been confirmed in the carbothermal processing with a CO rich atmosphere,28-30 in which the transition proceeds progressively through reaction between W2C and atomic carbon or CO. The metal loadings of the WCx/CNTs prepared from microwave-assisted MOCVD were analyzed using a TG technique (Figure 3). From the TG data, the obtained W loadings on CNTs

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Figure 4. Transmission electron micrographs of the WCx/CNTs from microwave-assisted MOCVD for 10 (a and d), 20 (b and e), and 30 min (c and f).

Figure 5. Transmission electron micrographs of the WCx/CNTs from thermal MOCVD.

of 4.5, 8.7, 13.2, and 18.3 wt % are very consistent with the theoretical loading of 5, 10, 15, and 20 wt %, respectively, indicating that microwave-assisted MOCVD is an efficient way to deposit the precursor on the CNTs without pretreatment. To assess the distribution and the dispersion of tungsten carbides on the CNTs, N2 physisorption was carried out. The BET specific surface areas of the WCx/CNTs with different tungsten loadings show that the surface area of the samples decreases from 87 to 70 m2/g with the increase of tungsten loadings from

4.5 to 18.3 wt %, indicating that the formed carbides are located on the out walls of CNTs and do not block the end of the CNTs. The particle sizes of the WCx/CNTs are confirmed using transmission electron microscopy (TEM) measurements. The TEM images and particle size distributions of the WCx/CNTs by microwave irradiation for 10 (Figure 4a), 20 (Figure 4b), and 30 min (Figure 4c) showed that uniform nanostructured tungsten carbides with the particle sizes of 2-5 nm were evenly dispersed on the outer surface of CNTs, which is in good agreement with the value from XRD. As the time increases from 10 to 30 min, the particles tend to become larger from 2.79 to 3.44 nm. However, the sample by microwave irradiation 3 min showed that there is no tungsten carbide on some CNTs, which maybe due to the fact that the short time is not enough for tungsten carbonyl to sublimate and decompose completely. High resolution TEM images of the WCx/CNTs by microwave irradiation for 10 (Figure 4d), 20 (Figure 4e), and 30 min (Figure 4f) showed that tungsten carbide particles were uniform and spherelike, but a few of particles were covered by amorphous carbon. On the contrary, the particles on the sample from thermal MOCVD had a much lower dispersion and were not uniform, spanning from several nanometers to tens of nanometers (Figure 5). Compared to samples from the direct thermal decomposition, carbothermal hydrogen reduction, alkaline reduction in solution, and temperature-programmed reduction, the distribution and the particle sizes of tungsten carbides on CNTs were more uniform and smaller in the microwave-assisted MOCVD. When the preoxidised CNTs were used to prepare the supported tungsten carbide particles by the microwave-assisted MOCVD, a few of

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4. Conclusion In conclusion, the microwave-assisted MOCVD can be efficiently used to synthesize highly uniform and well-dispersed nanostructured tungsten carbide particles of about 2-5 nm on CNTs without preoxidation. The preoxidation of CNTs is not necessary for preparation of nearly monodispersed nanostructured tungsten carbide particles. The tungsten carbide particles with 2-5 nm on CNTs can be formed several minutes, and the particle sizes increase with the increase of microwave irradiation. The prepared WCx/CNTs nanocomposites are active catalysts for hydrazine decomposition and exhibit high selectivity to hydrogen, indicating that nanostructured tungsten carbides on CNTs are an inexpensive and promising alternative to the noble metal catalysts. Thus, microwave-assisted MOCVD is of great potential in the controlled synthesis of supported carbide as catalytic materials, and the resultant nanocomposites are worth further exploring in electrocatalysis and hydrogen-involving reactions. Acknowledgment We gratefully acknowledge the financial support provided by the Program for New Century Excellent Talents in Universities of China (No. NCET-07-0133) and Dalian Foundation for Science and Technology, Liaoning Province (No. 2007J22JH008). Figure 6. Conversion and H2 selectivity of hydrazine decomposition over WCx/CNTs and CNTs.

Literature Cited

tungsten carbide particles were observed. This is different from conventional preparation route for metal particles, where the preoxidation of CNTs is a necessary step to provide anchoring sites.38 Further studies are in progress to determine the detailed mechanism of the formation of the tungsten carbides in the microwave-assisted MOCVD. The catalytic activity and selectivity of the microwave-assisted MOCVD produced WCx/CNTs were examined by hydrazine decomposition at atmospheric pressure in a fixed-bed reactor. The conversion and H2 selectivity of hydrazine decomposition over the WCx/CNTs with 18.3 wt % W loading and CNTs were shown in Figure 6. In the case of CNTs, a mass of hydrazine decomposition begins at up to 200 °C and much higher temperature is needed for further decomposition. The result is similar to the hydrazine decomposition by itself, indicating the CNTs have no activity for hydrazine decomposition and have a low selectivity to hydrogen. For the WCx/CNTs catalyst, the hydrazine conversion increases with an increase in reaction temperature. When the temperature is up to 120 °C, the hydrazine conversion reaches 100% and the products mainly are ammonia and nitrogen, which confirms that the hydrazine decomposition mainly occurs by the route: 3N2H4 f 4NH3 + N2, but not N2H4 f 2H2 + N2. This is in good agreement with the result observed in iridium catalysts.1-3 The selectivity to hydrogen is around 5% when the reaction temperature is below 500 °C. The selectivity to hydrogen begins to rise at 500 °C and then increases sharply with a further increase of the reaction temperature, which is very consistent with ammonia decomposition. At 700 °C, the selectivity to hydrogen is close to 100%. The temperatures corresponding to complete conversion and 100% H2 selectivity are quite comparable to literature values for WC catalysts. The performance for the hydrazine decomposition was similar to that over iridium catalyst,1-3 indicating that tungsten carbides are promising alternative to the noble metal catalysts for space applications.

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ReceiVed for reView October 21, 2008 ReVised manuscript receiVed January 8, 2009 Accepted February 3, 2009 IE801591X